The present invention relates to a bidirectional optical communication device and a bidirectional optical communication apparatus for carrying out bidirectional transmission and reception of an optical signal, and more particularly relates to a bidirectional optical communication device and a bidirectional optical communication apparatus for use in domestic communication, communication among electronic equipment, and in LAN (Local Area Network) with a multimode optical fiber such as a plastic optical fiber as a transmission medium.
Accompanied by progress of information oriented society, network technology with use of optical fibers is attracting attention. Particularly with recent progress of low-loss broadband POF (Plastic Optical Fiber), application of optical fibers to domestic communication and LAN is proceeding. In optical communication apparatuses for transmitting and receiving signal light having an identical wavelength with use of an optical fiber as a transmission medium, the leading system has been a full duplex system with use of two optical fibers. However, use of two optical fibers brings about such problems as difficulty in downsizing of optical devices and increase in cost of optical fibers with increased transmission distance. Accordingly, there has been proposed a bidirectional optical communication device for carrying out a full duplex optical communication.
In such bidirectional optical communication device with use of one optical fiber, transmission and reception are carried out in the same optical fiber, which makes it important to prevent interference of transmission light and reception light. Main causes of the transmission light interfering the reception light include:
(1) transmission light reflecting on the end face of an optical fiber when going into the optical fiber (hereinafter referred to as “near-end reflection”),
(2) transmission light traveled through an optical fiber reflecting on the end face of an optical fiber when going out of the optical fiber (hereinafter referred to as “far-end reflection”),
(3) reflection from a remote bidirectional optical communication device (hereinafter referred to as “remote module reflection”), and
(4) internal scattered light inside a bidirectional optical communication device (hereinafter referred to as “stray light”).
Among the causes (1) to (4), the far-end reflection of (2) is determined by the shape of the end face of an optical fiber, and therefore it is difficult to control the far-end reflection by the structure of a bidirectional optical communication device. For example, in a plastic optical fiber, if having a flat end face perpendicular to an optical axis, outgoing light from the optical fiber gains far-end reflection of about 4% due to difference in refractive index between a core and air. Accordingly, there is known a method for reducing the far-end reflection by processing the end face of the optical fiber. As the shape of the end face of the optical fiber for reducing the far-end reflection, a curved surface such as a sphere and an ellipsoid is known effective as disclosed in Japanese Patent Laid-Open Publication HEI No. 11-72622.
Conventionally proposed bidirectional optical communication devices enabling full duplex communication with one optical fiber involve a method for displacing an incidence position of transmission light from the center of an optical fiber end face in radial direction and disposing a light receiving element in a position free from incidence of light reflected from the optical fiber (near-end reflection) as disclosed in Japanese Patent Laid-Open Publication HEI No. 11-27217, Japanese Patent Laid-Open Publication HEI No. 11-237535, and Japanese Patent Laid-Open Publication HEI No. 11-352364. This method will be described with reference to a bidirectional optical communication device shown in FIG. 14.
In FIG. 14, transmission light 313 emitted from a light emitting element 304 is collected by a lens 306 while an optical path thereof is changed by a riser mirror 308 so as to enable incidence in a position displaced from the center of the end face of an optical fiber 302. Reception light 309 emitted from the optical fiber 302 is coupled to a light receiving element 305 disposed opposed to the optical fiber 302. The transmission light 313 whose optical path is changed by the riser mirror 308 goes into the optical fiber 302 with an incline from periphery to the center of the optical fiber 302. Consequently, reflected light 317 reflected by the optical fiber 302 is directed to periphery of the optical fiber 302, and an area other than the light receiving element 305 is radiated therewith, which enables prevention of interference due to near-end reflection. Further, decreasing an numerical aperture (NA) of the transmission light 313 makes it possible to decrease spread of the reflected light 317, thereby ensuring prevention of near-end reflection.
However, applying the bidirectional optical communication device shown in FIG. 14 to the case of using an optical fiber whose end face is in the shape of a curved surface such as a sphere cause a following problem.
The incident transmission light to the optical fiber is refracted by difference in refractive index between the core of the optical fiber and outside (air). For example, in an optical fiber having a sphere end face, transmission light whose incidence position is displaced from the center of the optical fiber end face in radial direction is refracted toward the central direction of the optical fiber, and a refractive angle thereof becomes larger as incidence position of the transmission light to the optical fiber goes nearer to the periphery of the optical fiber. In this case, as shown in FIG. 15, transmission light 8 inside an optical fiber 2 is composed of a dominant component having a large angle against an optical axis of the optical fiber 2 (higher mode) and a fractional component having a small angle against an optical axis of the optical fiber 2 (lower mode). Normally, the mode of the transmission light 8 is converted during traveling through the optical fiber 2, so that distribution of outgoing light from the optical fiber 2 is determined only by characteristics of the optical fiber 2 without being influenced by the state of incident light. Consequently, the transmission light 8 can go into the optical fiber 2 only with consideration to the condition of coupling the transmission light 8 to the optical fiber 2 (decreasing the numerical aperture (NA) of the transmission light 8 based on the numerical aperture NA of the optical fiber 2). However, in the recent years, necessary transmission capacity has been enlarged in inter-equipment transmission, and communication with use of an optical fiber has started to be applied to the case where a transmission distance is as short as about 1 m, causing a new problem. The problem is that a short transmission distance prevents sufficient mode conversion, and outgoing light thereof is largely influenced by the state of incident light. For example, as shown in FIG. 15, when an incidence position of the transmission light 8 is displaced from the center of the sphere end face of an optical fiber 2 in radial direction, radiant intensity of the outgoing light shows, as shown with a solid line in FIG. 16, a ring-shaped distribution small in quantity of light radiated from the central part of the optical fiber and large in quantity of light in the peripheral part thereof. The influence is particularly large in the case of a large-diameter optical fiber with a number of modes such as POF.
In the case where a transmission distance is long or in the chase where transmission light goes into the middle of the optical fiber, as shown with a broken line in FIG. 16, there is obtained a distribution large in radiant intensity from the center of the optical fiber. Thus, remarkable change in the distribution of radiant intensity of outgoing light from the optical fiber due to the transmission distance or incidence condition of the transmission light causes considerable deterioration of reception efficiency in the either case depending on disposal of reception light, resulting in exceeding of a dynamic range. Particularly in the bidirectional optical communication device carrying out full duplex communication with one optical fiber, the incidence position of the transmission light is displaced from the center of the optical fiber end face in radial direction, which enlarges fluctuation of the reception efficiency and limits the transmission distance.
Furthermore, since a method for reducing remote module reflection has not been disclosed, the conventional bidirectional optical communication device suffers interference due to the remote module reflection.